feat(D): OCC mesh attribute extraction + Blender smooth shading integration

Migration 039: cad_files.mesh_attributes JSONB column.
domains/products/tasks.py: extract_mesh_attributes Celery task using pythonOCC.
still_render.py + turntable_render.py: _apply_mesh_attributes() sets auto-smooth
based on curved_ratio and topology threshold from OCC analysis.
render_blender.py: passes --mesh-attributes JSON arg to Blender subprocess.
render_still_task: loads mesh_attributes from DB and passes to renderer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/rendering/tasks.py

@@ -48,6 +48,7 @@ def render_still_task(
     denoising_prefilter: str = "",
     denoising_quality: str = "",
     denoising_use_gpu: str = "",
+    mesh_attributes: dict | None = None,
 ) -> dict:
     """Render a STEP file to a still PNG via Blender subprocess.
 
@@ -83,6 +84,7 @@ def render_still_task(
             denoising_prefilter=denoising_prefilter,
             denoising_quality=denoising_quality,
             denoising_use_gpu=denoising_use_gpu,
+            mesh_attributes=mesh_attributes or {},
         )
         logger.info(
             "render_still_task completed: %s → %s in %.1fs",